Carbon monoxide is usually obtained by separation from synthesis gases produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. In addition to carbon monoxide, these gases contain hydrogen and methane. It is well known to separate carbon monoxide from such mixtures by a cryogenic separation process in which carbon monoxide is removed by a low temperature scrubbing step using liquid methane in a wash column to provide a CO-loaded methane containing some, typically 3-4%, hydrogen. Residual hydrogen is removed from the CO-loaded methane in a stripping column to meet the required carbon monoxide product specification and the hydrogen-stripped CO-loaded methane is separated into a CO-rich vapor and a methane-rich liquid in a fractionation column. Usually, a portion of the CO-rich vapor is recycled to provide a heat pump stream and a portion of the methane-rich liquid is recycled to provide the methane wash liquid.
U.S. Pat. No. 3,813,889 discloses such a process in which the total hydrogen-stripped CO-loaded methane is expanded and then warmed and partially vaporized before being introduced into the CO/methane fractionation column as a single feed. The heat duty required to warm and partially vaporize the fractionation column feed stream is provided by cooling and condensing a gaseous CO recycle heat pump stream.
U.S. Pat. No. 5,133,793 describes a similar process in which the hydrogen-stripped CO-loaded methane is sub-cooled, expanded and split into three sub-streams. The first substream is fed directly to the CO/methane fractionation column as a liquid at about its bubble point; the second substream is vaporized and warmed up to its dew point before feeding to said column; and the third substream is vaporized and warmed to a temperature intermediate said bubble and dew point temperatures and then fed to said column. The heat duty required to vaporize and warm the second and third substreams is provided by heat exchange with the total sub-cooling hydrogen-stripped CO-loaded methane and with cooling and partially condensing gaseous feed.
U.S. Pat. No. 4,311,496 discloses a process in which, prior to the methane wash, a gaseous feed containing hydrogen, carbon monoxide and at least 25 vol % methane is subjected to a continuous partial condensation at a higher temperature than the methane wash to provide a gaseous phase fraction and a condensed liquid phase fraction. The gaseous phase fraction is subjected to the methane wash and hydrogen and carbon monoxide are stripped from the liquid phase fraction. Partially condensed overhead vapor from the stripper and bottoms liquid from the methane wash are fractionated into a CO-rich overhead vapor and a methane-rich bottoms liquid. The CO-rich vapor supplies CO to a recycle heat pump stream providing reboil and reflux to the fractionator and the methane-rich liquid supplies wash liquid to the methane wash.
An object of the present invention is to obtain additional pre-heat in an efficient manner to reduce the overall separation energy requirements of the separation process.